The correct folding of nascent polypeptide chains into active proteins is a fundamental biological process. All cells contain a 70K heat shock protein (Hsp70) believed to play a central role in the protein folding pathway. As its name implies, Hsp70 is a protein which is induced when cells are exposed to greater than optimal temperatures. However, many members of the Hsp70 family are not only required for viability at increased temperatures, but arc also required for growth under normal conditions. The goal of this project is to determine if Hsp70s are indeed required for protein folding in vivo by studying the two cytosolic Hsp70 proteins (SSA and SSB) found in Saccharomyces cerevisiae. By using conditional mutants, the requirement for Hsp70s in protein folding will be monitored by measuring the synthesis of active reporter proteins. In addition, the biological difference between the SSA and SSB proteins will be assessed. The ability of each of these two Hsp70 proteins to refold denatured mature proteins will be measured. The requirement for either Hsp70 protein in the folding of nascent polypeptides in an in vitro yeast translation system will be determined. Finally, the difference in peptide recognition between SSA and SSB will be identified.